This invention concerns leak testing and more particularly leak testing of cavities in test parts by flow rate measurement. Such leak testing procedures have heretofore been practiced by placing a source of fluid under pressure in communication with the cavity to be tested, and measuring the flow rate to the cavity, after allowing stabilization of the flow to a steady state flow condition. Under these circumstances, the flow to the cavity after stabilization will equal the flow from the cavity, i.e., the leakage flow. If no leak is present, then the rate of fluid flow to the cavity from the pressure source will be zero.
In more refined versions of the method, a prefill fluid circuit is employed in which a high capacity passage is initially placed in parallel communication between the fluid pressure source and cavity together with a small capacity passage within which the flow measurement takes place. This allows a relatively large volume of flow to the cavity initially to rapidly pressurize the cavity. The second low capacity flow passage is solely in communication with the pressure source and cavity after the prefill interval, and flow through the lower capacity passage is measured after the establishment of steady state conditions to evaluate the leakage of the cavity. This two-stage pressurization allows a relatively rapid test cycle, since pressurizing the cavity with fluid under pressure through the low flow rate capacity passage would take an excessive interval of time.
In a still more refined version of the method, an even more rapid test cycle is achieved by measuring the flow rate well prior to the establishment of steady state flow conditions and extrapolating the flow rate to steady state conditions.
This is made possible by determining the earliest point in the pressurization cycle after which the flow rates become repeatable for a given leak rate. It has been observed that a repeatable correspondence between transient flow rates and the steady state flow rates allows accurate extrapolation from the transient but repeatable flow rates occurring earlier in the test cycle to the steady state flow rates.
It has been found that if the temperature of the test part varies significantly over the ambient temperatures and the normal temperature for which the system is calibrated, a wide variation in test results occurs in practicing this leak test method.
This is particularly true for the high speed testing, which does not allow for equalization of the part temperature with the ambient temperature.
Such leak testing is sometimes done with the parts well above or below ambient temperatures as a result of processes performed on the test part just prior to the leak test. It is often not feasible to delay testing until the parts are cooled or warmed to ambient temperatures.
Such leak testing is often conducted on parts of widely varying configuration, which parts are often constructed of many different materials, these factors affecting the way temperature differences change the results of the leak tests. The test pressure and the cycle times are also varied constantly in leak testing different parts, which also affects the change in results due to temperature shifts of the parts.
Disclosed in U.S. Pat. No. 4,272,985 is a pressure decay type leak test system, which employs a method for compensating for temperature variations in the test part. This method requires an additional stage in the test cycle, however, which lengthens the time for completing the leak test. Since such leak testing must be done as part of the production process, lengthening the test cycle time will reduce productivity of the production process.
Also, the method disclosed in that patent is relatively complex.
Accordingly, it is an object of the present invention to provide temperature compensation for leakage testing of test parts to compensate for the effects of significant variations in temperatures of the test parts from ambient temperatures, as the test parts undergo the leak testing.
It is a further object of the present invention to provide such temperature compensation method which is directly applicable to flow rate type leakage testing and particularly adapted to the two-stage type flow rate testing as described above with a prefill high capacity flow coupled with a subsequent low flow rate to the test cavity.
It is still another object of the present invention to provide a simple and reliable method for providing such temperature compensation in leak testing of parts, which is quickly adaptable to parts of widely varying configuration and material, and to tests of widely varying test pressures and cycle times.